Focused ion beam (FIB) sources generally extract ions from an ion source and accelerate and focus the extracted ions into an ion beam. The focused ion beam is useful for performing surface and structural analysis or modification, such as precision cutting and material deposition. FIB sources are of increasing and critical importance to the semiconductor industry, as FIB sources significantly reduce the time and costs concomitant with integrated circuit design, development, fabrication, evaluation, and repair.
The critical parameters of FIB sources include the diameter of the ion beam or spot size, the ion current density, and the energy range or spread of the beam. Obviously, focused ion beams are employed where a small spot size is desirable. The ion current density is the brightness of the focused beam and is measured in amps/cm.sup.2. The energy range of the ion beam is described by .DELTA.E/E, where .DELTA.E is the energy spread of the ion beam and E is the extraction energy of the ion beam source. An ion beam having a narrow energy range (low .DELTA.E) is more easily focused into a small spot size than an ion beam having a large energy range. Additional considerations are the average energy of the ion beam, the available target materials for producing the ions comprising the beam, the ability to generate a pulsed ion beam, as opposed to a continuous wave (CW), and the ability to produce particles in addition to ions, for example, molecules, atoms, or ion clusters within the beam.
For applications in the semiconductor industry, it is desirable to have a FIB source capable of producing an ion beam having a submicron spot size, a high current density, and a narrow energy range. A submicron spot size is advantageous due to the diminutive complex structures of the microelectronic devices, e.g., integrated circuits (ICs). An ion beam having a high current density is desirable for better sputtering, imaging, and cutting in surface analysis and modification applications. A variety of target materials may be used to generate the ions, and there is new interest in the desirability of the presence of molecules, atoms, or ion clusters within the beam to allow sputtering of larger surface species for surface analysis and modification. A pulsed ion beam having a high rate of repetition would be particularly advantageous for surface analysis applications, since instruments used for detecting sputtered or back scattered ions require a pulsed ion source, for example, Secondary Ion Mass Spectrometers (SIMS). An adjustable average energy range allows the spot size of the ion beam to be changed to desired dimensions.
Available FIB sources include liquid metal gun ion sources (LMIGSs) and gas field ionization sources. LMIGSs produce ion beams by applying a strong electric field to the end of a needle or capillary of liquid metal, typically either pure metals with low melting points (e.g., Ga, Li) or eutectic alloys (e.g., AuSi, CoNd, CoGe), which results in the liquid forming a sharp tip (Taylor cone) where ion emission occurs. U.S. Pat. No. 4,639,301 issued to Doherty, et al. describes a submicron focused ion beam using a LMIGS. Although LMIGSs produce high current ion beams (about 10 amps/cm.sup.2) with submicron resolution, the ion beams have large energy ranges or spreads which cause chromatic aberration of the ion beam, limiting minimization of the spot size. LMIGSs are also generally limited as a continuous wave (CW) ion beam source, however, "pulsed" ion beams may be generated with some amount of difficulty using an aperture and a scanning technique.
Gas phase field ionization emitters produce ion beams from a volume of gas, such as argon, krypton, oxygen, or nitrogen, rather than from a liquid metal needle. Ions are produced from gas molecules, by known methods including photo-ionization, and that traditional extraction optics are used to form the ion beam. Since the source of the ion beam is a volume, as opposed to a plane or small surface area, the brightness or current density of the extracted ion beam is low (about 0.1 amps/cm.sup.2). Although gas phase ionization sources have medium energy spreads (less than LMIGSs), they are not capable of providing focused high current beams having the narrow energy ranges required by many of the semiconductor applications. The average spot size for gas phase field ionization sources is as small as about 1 micron.
Most FIB systems use computer control to guide the ion beam, whether in a raster pattern (scanning from side to side in lines from top to bottom) or in a vector pattern (scanning over a selected area of a substrate). Also, lens optics and deflecting plates are used to control chromatic aberration and spherical aberrations of the beam. Chromatic aberration is an increase in the beam size due to the velocity dispersion of the beam. In other words, the beam must be nearly monoenergetic, wherein the ions have the substantially the same energy, for the focusing lens to effectively focus the beam into a subnucron spot size. Therefore, the chromatic aberration is a function of the ion beam's energy spread (.DELTA.E/E), and a narrow energy spread is most beneficial for generating an ion beam with a submicron spot size. Spherical aberrations are an increase in the beam size due to a variation in the beam particles' focal length from the central axis of the beam. If the ion beam begins from a source having a large surface area or volume, then producing a small spot size is more difficult than producing a small spot size from a beam originating from a small surface area or small volume. In other words, an ion source that creates ions in a very small volume has very high brilliance (large concentration of ions per unit volume). Chromatic aberrations are more limiting to final beam size than spherical aberrations.
Advantages of using FIB sources are a computer controlled beam, no mask requirement, submicron spot size, minimization of diffraction effects, less back scattering, higher resolution, and very accurate detection of surface features. Disadvantages include the need for a reliable high current ion source having a narrow energy spread, rather than the very large energy spreads of the currently available high current focused ion beams.
The present invention provides a reliable FIB source for generating a pulsed ion beam having a high current density, narrow energy spread, and submicron spot size by using laser energy. Surprisingly, an ion beam produced by specific control of laser ablation has a narrow energy spread and a small angular distribution (the initial trajectory of the sputtered ions are substantially along an axis perpendicular to the surface of the target material). The focused ion beam is intrinsically pulsed by employing laser technology, and the pulsed nature of the ion beam has significant advantages in surface analyses applications over continuous wave FIB sources. The average energy of the generated ion beam is also adjustable over a wide range. The present FIB source advantageously produces a variety of primary beam charged particles, including molecular ions and ion clusters, and the ion source material may be any solid or liquid matter vaporized by laser energy. The present FIB source is intrinsically pulsed and requires no cross beam electric fields, eliminating tailing difficulties associated with pulsed sources. The design of the present FIB source is very simple and economical, especially since powerful and inexpensive lasers are readily available.
Therefore, in view of the above, a basic object of the present invention is to provide a method and apparatus for producing an ion beam having a high peak current and narrow energy spread on a target area in the submicron range by using laser radiation.
A further object of this invention is to produced the high current ion beam in a pulsed manner.
Another object of the present invention is to produce an ion beam capable of generating a variety of particles for analysis, including atomic ions, molecular ions, and ion clusters.
Yet another object of the invention is to use a variety of solid or liquid materials to produce the primary ion beam, including, but not limited to: Al, Au, Ga, Si, C.sub.60, Cs, C, B, and P, and alloys thereof.
Yet another object of the invention is to produce an ion beam having an ion current that is adjustable over a wide energy range.
Additional objects, advantages, and novel features of the present invention will become apparent from the description which follows and/or will become apparent to those skilled in the art upon examination of the following or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of instrumentation and combinations particularly pointed out in the appended claims.